1. Field of the Invention
This invention relates to distance measuring equipment, and is particularly concerned with distance measuring equipment wherein two images of an object are received along two different optical paths on photoelectric sensor arrays each; two image data rows which represent a luminous intensity distribution in the object are obtainable by quantizing image signals operated by the images are compared with each other, and a distance to the object is measured or estimated from a mutual shift rate of both signal rows which is required for coordinating both signal rows at high correlation.
2. Description of the Prior Art
The distance measuring equipment described above has been known for a fairly long time, however, there is currently provided a device wherein no moving part is totally incorporated; that is, a full electronic type equipment, which is duly appreciated as miniature, cheap and high precision distance measuring equipment.
A principle of this kind of equipment is shown in FIG. 1 and FIG. 2. In FIG. 1, a light emitted from an object 1 subjected to measurement of a distance d thereto, or, for example, the sunshine reflected from the object is incident on a pair of small lenses 2, 3 along two different optical paths 4 and 5. These lenses having a short focal distance f, are incorporated in an optical instrument, having a base distance b therebetween. The object 1 has a luminous intensity distribution given in two triangles as illustrated, and images 7, 8 of the object having such luminous intensity distribution are formed on a focal plane 6 through the small lenses 2, 3. To make the description easy to understand, a center of the object 1, namely a center 1C of the luminous intensity distribution faces in front of the small lens 2, a center 7C of the image 7 on the focal plane 6 through the small lens 2 comes at a position indicated by 70, and the image center position 70 does not change regardless of a change in the distance d to the object 1. On the other hand, as will be easily understood a center 8C of the image 8 formed through the small lens 3 is kept at a position 80, on the focal plane 6 facing to the lens 3, when the distance d to the object 1 is infinite. However, it is dislocated leftward in the illustration in accordance as the distance d to the object 1 becomes short, and thus the center 8C of the image comes to a position 81 apart from the original position 80 by a distance x on the focal plane 6 as illustrated.
Now, there are provided, on the focal plane 6, photosensor arrays 10, 11 for receiving the images 7, 8 of the object 1 through the small lenses 2, 3 each. The photosensor arrays 10, 11 generally consist of mutually different and m, n pieces of photoelectromotive force elements or photosensitive resistance elements each, and each element of the arrays generates an electrical signal relating or proportional, for example, to the quantity of light received as shown in FIGS. 2(a), 2(b). Now, if the distance x of the above-mentioned dislocation can be measured on some means, then the distance d to the object 1 will be obtainable through an expression: EQU d=b.multidot.f/x
according to a simple principle of triangular surveying.
Then, a signal obtained from each element of the photosensor arrays 10, 11 has an analog value shown in FIGS. 2(a), 2(b), and a distribution of the output signal along each photosensor array has a step-like pattern as illustrated. The analog value can be used straight for obtaining the distance x of the above-mentioned dislocation, however, it is normally quantized to a digital value for simplifying and improving precision of an electronic circuit. As the simplest means for quantization, the analog value is compared with an appropriate threshold voltage Vt as shown in FIGS. 2(a), 2(b), then the analog value greater than the threshold value Vt is given at "1" and that of less than the threshold is given at "0", which are converted into one-bit digital values as shown in FIGS. 2(c), 2(d). Next, distributions of the digital values along both the photosensor arrays 10, 11 which are shown in FIGS. 2(c), 2(d) are compared with each other through the electronic circuit, thereby distance x of the dislocation is converted to a value obtained on the sensors. The distribution of the digital values indicated by a solid line in FIG. 2(c) corresponds to the case where the distance d to the object 1 is infinite and hence the dislocation x is 0, from which it is understood that the measurement of the distance d may result in obtaining the number of the elements by which the distance x on the photosensor array shown in FIG. 2(d) is expressed.
Then in FIG. 1, the description refers to the case where an optical axis of a finder (not illustrated) for selecting the object 1 subjected to determination of the distance d thereto coincides with the optical axis of the small lens 2, namely the small lens 2 faces right to the object 1 as described hereinabove, however, optical axes of the finder and the small lens do not coincide generally with each other, needless to say. Assuming now the finder comes intermediately of the two small lenses 2, 3, the images 7, 8 on both the photosensor arrays 10, 11 will be dislocated right by distance X.sub.1 and toward left by distance X.sub.2 from the original position when the object 1 is infinitely located. But in this case, the distance d to the object 1 can also be measured by the same relational expression as described above by considering x=x1+x2. Therefore the measurement of the distance d may also result in obtaining the dislocation x of images on both the sensor arrays, after all.
A circuit of the distance measuring equipment according to the prior art on the above principle is shown in FIG. 3. Two shift registers 12, 13 are shown in the drawing, and digital signals shown in FIGS. 2(c), 2(d), which are obtained by quantizing the output signals shown in FIGS. 2(a), 2(b) coming from the photosensor arrays 10, 11 shown in FIG. 1 are through an analog-digital converter (not shown), are stored in the shift registers corresponding to the sequence of the arrays of the photosensors. After the image signals are written in the shift registers 12 and 13 as above, a shift signal is applied to control terminals CTR of the shift registers 12 and 13 from a timing control unit 14, and the data of the image signals stored in the shift registers 12 and 13 are sequentially outputted from output terminals "out", being synchronized with each other stage by stage of the shift registers. The output of the shift registers 12 and 13 are returned to an input terminal "in" each and restored. An exclusive NOR circuit 15 makes "1" when the outputs from the shift registers 12 and 13 coincide and "0" when not coincident. A counter 16 counts the number of times when "1" is generated by the exclusive NOR circuit 15.
Now, assuming that the numbers of photosensors in the photosensor arrays 10, 11 shown in FIG. 1 are m and n pieces respectively, the even m and n pieces of image data are stored in the shift registers 12, 13 respectively, and m&lt;n, then, when the data have been outputted m times from the beginning. All the data stored in the shift register 12 have been compared with the first m pieces of the data of the shift register 13. The counter 16 has counted how many bits are coincident as the result of the comparison therebetween under the state where they are not dislocated each other that is, the number of dislocations is 0. Further in such state, the contents stored in the shift register 12 have made a round to initialization, and the contents stored in the shift register 13 have circulated in dislocation m bits rightward. The contents of the counter 16 are then stored in a maximum coincidence storage circuit 17. Further, only the shift register 13 is shifted by (n-m+1) bits according to an indication from the timing control unit 14, and the counter 16 is cleared. The contents of the shift register 13 are taken dislocated 1 bit rightward from the initialized state according to the shift by (n-m+1) bits. A counter 18 is that of counting how many bits the contents stored in the shift register 13 are shifted rightward from the initialized state, which is stepped forward whenever the above-mentioned data comparison of the shift registers 12 and 13 are over. With the state wherein the contents stored in the shift register 13 have been shifted 1 bit rightward from the first initialized state as mentioned, the second comparison is carried out by shifting the data of the shift registers 12 and 13 m times rightward in sequence likewise. At the stage in which the second comparison has been over, the contents C1 of the counter 16 are compared for dimension with contents C2 of the maximum coincidence storage circuit 17 on a comparator 19, and if C1 is greater than or equal to C2, C1 is written in the maximum coincidence storage circuit 17. Then, contents S1 of the shift counter 18 are written in a dislocation number storage circuit 20 concurrently therewith. Only the shift register 13 is shifted by (n-m+1) bits rightward thereafter, and the counter 16 is cleared. From that time on, comparison of the stored contents between the shift registers 12 and 13, comparison of the contents between the counter 16 and the maximum coincidence storage circuit 17 and subsequent rewrite of the maximum storage circuit 17 and the dislocation number storage circuit 20, shift of the data of the shift registers 13 at (N-m+1) times, and clearance of the counter 16 are repeated at a predetermined number of times. At the time when the repeat has been over, the situation is such that a maximum incidence number as the result of having approved a coincidence of the contents of the shift register 12 with a part of the contents of the shift register 13 is stored in the maximum coincidence storage circuit 17, and a relative dislocation number between the shift registers 12 and 13, namely the dislocation rate x to obtain which is shown in FIG. 2(d) is stored in the dislocation number storage circuit 20. The timing control unit 14 latches contents of the dislocation number storage circuit 20 on an output latch 21 as a final step of the operation, thereby generating as a distance signal externally.
The distance measuring equipment constructed as above, which is full electronic and free from a moving part is found useful for its being miniaturized, cheap and high in distance measuring precision, however, there yet remain various problems when the equipment is actually operated. That is to say, a distance measuring result is erroneous, in most cases, where the object subjected to measurement of a distance thereto has a repetition of simple patterns such as stripes and checks. The cause is relevant to a principle on the detection of a coincidence of two images. In the case of such patterns, a dislocation number for making two images coincide may be allowed to exist plurally always. In such case, according to the conventional system shown in FIG. 3, the dislocation number whereby the count C2 of the maximum coincidence storage circuit 17 is maximized is merely obtained, therefore, if there exists plurally the dislocation number to make the two images coincide, the distance is capable of being measured from the dislocation number corresponding to a maximum coincidence point found by chance at first. Accordingly, the conventional measuring equipment may assume the distance based on the result corresponding to the farthest distance or the nearest distance of the plurality of dislocation numbers whereby the two images can be made to coincide to be a measured result, and thus forcusing a camera thereon, for example, may bring a blur on the image consequently. A similar problem occurs where the object to be subjected to a distance measurement is very dark. In such case, since an image output signal from the photosensor is low it is close to the threshold value Vt when the signal is quantized according to the method given in FIGS. 2(a), 2(b). Therefore, a pattern distribution of the quantized image data is too scanty to obtain the maximum coincidence sharply. The maximum coincidence point becomes obscure inevitably, or there may occur a case where the maximum coincidence point exists plurally as mentioned hereinbefore, and thus an error is easy to arise on the distance measurement.
One of the other problems refers to the case where the object to be subjected to distance measurement is dark in luminosity to take, for example, a minus EV value, and in this case since the quantity of light incident on the photosensor arrays 10, 11 of FIG. 1 is little, an output signal value from each photosensor in the arrays becomes low. Therefore, the signal does not reach or comes barely close to the threshold value Vt at the time of signal quantization shown in FIG. 2, and thus a result obtained through measuring the distance by means of such quantized digital signal value will not be reliable entirely. Of course, the matter can be settled by using an optical signal storage type sensor for the photosensor, however, since a considerably long time is required for storing the signal in this case, it can no more be applied to a video camera wherein it is not allowed to take considerably long time for measuring a distance because an image is recorded continuously while the field of video is moving. Accordingly, the image pickup as keeping the time for distance measurement at a minimum requirement may be effected according to an erroneously measured result on distance and also under an erroneous focusing condition, thus bringing a blur on the image.
In case, on the contrary, that the object has a satisfactory brightness, but there is scarcely observed a contrast of lightness in the object, the distance measurement becomes difficult in view of the abovementioned measuring principle, and where the contrast is totally not observed, the measurement will be no more practicable. The reason is that a quantization of output signal of the photosensor at any threshold value in this case is not to obtain a pattern of the digital signal value necessary for the distance measurement.
On the other hand, there may be a case where an error arises on the distance measured result despite a brightness of the object. The difference in brightness in the field of view of a camera is exceedingly big generally, and there are many cases where the luminosity of a bright zone reaches 10.sup.6 as high as that of a dark zone, therefore it is difficult to change the threshold value thus extensively at the time of quantization. Even though the object is exceedinly bright and has a contrast of brightness therein, it exceeds a threshold value for quantization almost all, and thus information on the contrast of brightness will not be obtainable at all or, if any, it is considerably limited. In any case, a digital output from the analog-digital converter becomes poor in pattern information of "0" and "1", and even if the subsequent digital circuit operates correctly, a result obtainable through measuring the distance will be that of lacking accuracy. It was tried to increase information content of the brightness pattern by multiplying the output of the analog-digital converter in bit instead of such 1-bit information, however, such means is not necessarily effective where the property of the object is not proper as mentioned.
Besides, where a plurality of objects different in distance happen to come in the field of view of the photosensor arrays, or the object has a regular pattern in brightness such as, for example, stripes or checks, the digital circuit outputs a distance measuring data through searching minutely a point whereat pattern information from both left and right photosensor arrays will be at a maximum coincidence, therefore a case where the distance measured result is present plurally may arise, and thus a determination on which measuring data to take as a correct result will not be obtainable.